Copper(II) ions (Cu^{2+}) form a complex with lactic acid (C_3H_6O_3) as shown in the equation - HSC - SSCE Chemistry - Question 31 - 2023 - Paper 1

To find the equilibrium constant, we first need to determine the concentration of [Cu(C_3H_6O_3)_2]^{2+} at equilibrium. From the graph, an absorbance of 0.66 corresponds to a [Cu(C_3H_6O_3)_2]^{2+} concentration of 0.046 mol L^{-1}.

Next, we set up the reaction:

Cu^{2+}(aq) + 2C_3H_6O_3(aq) ⇌ [Cu(C_3H_6O_3)_2]^{2+} (aq)

We define initial concentrations and changes:

Initial	Change	Equilibrium
Cu^{2+}	0.056	-0.046
C_3H_6O_3	0.111	-0.092
[Cu(C_3H_6O_3)_2]^{2+}		+0.046

Now we can use the equilibrium concentrations to calculate the equilibrium constant (K_eq):

$K_{eq} = \frac{[Cu(C_3H_6O_3)_2]^{2+}}{[Cu^{2+}][C_3H_6O_3]^{2}}$

Substituting the values: $K_{eq} = \frac{0.046}{(0.010)(0.019)^2} = 1.3 \times 10^{4}$

Thus, the equilibrium constant for the reaction is approximately 1.3 × 10^{4}.